In manufacturing integrated circuits, many of the processing steps require vacuum processing, i.e., performing operations on a semiconductor wafer or a photomask within a vacuum chamber. For example, such processing steps as plasma etching, low pressure chemical vapor deposition and ion-implantation are commonly performed in a vacuum. Within the vacuum chamber, however, particles generated by chemical reactions and mechanical abrasions may contact and adhere to the surface of the semiconductor or photomask during processing, resulting in major yield losses.
The movement of such particles within the vacuum chamber can be attributed to two sources. First, the particles may fall on the wafers due to gravity. When gas is present in the vacuum chamber, the particles will be buoyed by the gas and fall very slowly; however, as the gas is removed to create a vacuum, the particles will begin to drop under the force of gravity at an increased speed. The second source of movement is caused by turbulence created by the venting of the vacuum chambers (adding gas to the vacuum chamber to bring the chamber to atmospheric pressure) or by pump-down (removing gas from the chamber to create a vacuum). Particles impelled by turbulence often follow a somewhat circular path within the chamber, as opposed to the essentially downward path of particles falling due to gravity.
Particles are introduced into the vacuum chamber through several mechanisms. Many particles are introduced by mechanical abrasion caused by movement of the equipment in the vacuum chamber, such as robotic arms or gate valves. As the surfaces of the mechanical apparatus rub against each other, particles are released into the chamber. Also, the venting gas may be impure, thus becoming a source of particles. Particles may also be formed as the result of chemical reactions in the chamber. Finally, residual particles may reside in the chamber prior to pump-down and vent.
If a small particle comes in contact with a semiconductor wafer or photomask surface in the vacuum chamber for processing, the particle will often adhere to the surface of the wafer or photomask. The primary forces of adhesion of small (less than 50 micrometer diameter) particles on a dry surface are van der Waals forces. The van der Waals forces of adhesion can increase as a function of time, due to particle and/or surface deformation which increases the contact area. Total forces of adhesion for micron-size particles exceed the gravitational force on that particle by factors greater than 10.sup.6. For particles with a diameter greater than 50 micrometers, electrostatic forces predominate.
A method of reducing defects due to particle adhesion is to align the surface of the wafer undergoing processing (the "active" surface) in a position other than horizontal and face-up. One such technique aligns the semiconductor wafer or photomask such that its active surface is vertical, i.e., perpendicular to the floor. In this orientation, the surface is less susceptible to particles falling by force of gravity. However, the surface is still in the path of particles moving under the force of turbulence.
Likewise, particle deposition on the wafer or photomask can be reduced by aligning the active surface in a face-down position, i.e. the active surface is positioned towards the floor of the vacuum chamber. In the face-down orientation, particles falling by force of gravity adhere to the back of the wafer or photomask where they are harmless. Once again, however, the active surface is susceptible to particles moving by force of turbulence.
From the foregoing, it may be seen that a need has arisen for a technology which prevents or reduces contact between small particles and a semiconductor wafer or photomask being processed in a vacuum chamber. Furthermore a need has arisen for technology capable of protecting against both falling particles and circulating particles.